1. Related Applications
This application is a continuation-in-part of my co-pending application Ser. No. 383,004 filed May 28, 1982 entitled "Method and Apparatus for Monitoring and Diagnosing Peripheral Blood Flow," now U.S. Pat. No. 4,569,355 (Feb. 11, 1986).
2. Field of Invention
The present invention relates to an apparatus for monitoring blood flow automatically by cooling a small area of skin through several cycles and monitoring the rate at which the internal blood flow returns the skin to normal temperature. The cooling device which the blood flow monitor uses also can be used for general cooling of the body in high-temperature environments.
3. Description of Prior Art and Patent Application
Parent U.S. Pat. No. 4,569,355 discloses that changes in blood flow can be monitored by cooling an area of skin terminating the application of cold and measuring the rate at which the internal blood flow raises the skin temperature. The rate of temperature change is a function of the efficiency of the circulatory system which supplies the capillaries in the immediate area of the skin. By cycling through alternating cooling and non-cooling cycles, involuntary constrictive muscles of the blood vessel arterioles begin to fatigue. The capillaries in healthy people return the skin temperature toward normal faster.
The parent application discusses one way of manipulating the data to yield a Cardio-Thermal Index or CTI. The CTI can be used for absolute readings for comparing circulatory efficiency in one person relative to another, or the individual can be his or her own control using the present invention to monitor changes in blood flow at a given "healthy" location as related to a vascular diseased area.
When one uses the device of the present invention to show changes over relatively long periods of convalescent time in a patient, the device can show the effectiveness of treatment and/or the prognosis. The present invention can be used for many other purposes. It can determine changes in peripheral vascular disease ("PVD"). As PVD increases, it is desirable to be able to diagnose these changes so that effective treatments or corrective surgery can be initiated or changed. Although present pharmacological treatments have been of limited value in treating PVD, new drugs are the subject of continuing current investigation, and it will be necessary to monitor subtle improvements in blood flow to test the efficacy of the new medications and to determine in the particular patients whether the drugs or other corrective measures are having their desired effect.
An unfortunate consequence of impaired blood flow is tissue damage, which, if it becomes too pronounced, can ultimately require limb amputation. The present invention can predict, based on a predetermined CTI, where on a limb an amputation should take place so that the area of the amputation receives sufficient blood flow for healing. It can also indicate that amputation is not needed and corrective surgery can be instigated negating the permanent and drastic effects of amputation. After amputation, the blood flow near the wound can be monitored non-invasively to determine whether the flow is sufficient for healing and whether the amputation site was properly chosen.
The accuracy of the method of monitoring blood flow is improved by precise controls of the cooling heat exchanger and its related instrumentation. The heat exchanger relies on injecting a low-boiling point liquid into a cooling chamber against the skin. The body heat boils the heat exchanger liquid thereby transferring some of its heat energy to vaporizing this liquid, which in turn cools the skin and the capillary bed. As set forth in the parent application, temperature recovery when there is no further liquid in the heat exchanger is slower in a person with PVD. In the parent application, a relatively complex structure was devised for accurately controlling injection of the low-boiling point liquid into the heat exchanger chamber. It is an object of the present invention to provide precise control for the amount of liquid entering the heat exchanger. Another object of the present invention is to provide this controlled cooling in a highly reliable instrument.
It is desirable that the cooling unit be small. The cooling unit in the parent application is designed to be relatively small, but the unit that is applied to the skin incorporates structure for determining the controlled amount injected into the heat exchanger. Thus, there are parts on the unit adjacent the skin that move. Without due care, the complex unit may lead to discomfort and inaccuracies in the readings as the movements of the unit and its own weight may have an effect on circulation at a localized test area.
It is not always necessary to locate all parts of the injector near the heat exchanger. The heat exchanger may be connected to parts of the injector through a line. The low-boiling point liquid in the line is essentially incompressible. A valve that opens at a predetermined, high pressure and is located in or near the heat exchanger can maintain the liquid in the line up to the predetermined pressure. Raising the pressure in the line by adding a controlled volume of liquid at the upstream end of the line causes the liquid at the downstream end of the line to be injected past a minute check valve into the heat exchanger. If the valve is upstream from the heat exchanger, the vapor pressure of the liquid causes liquid in the line between the valve and the heat exchanger to flow slowly into the heat exchanger. The slow flow of liquid lowers the pressure in the line, which may allow the liquid to boil in the line, which is undesirable. It is an object of the present invention to improve the valving to eliminate potential problems.
The liquid that is present at the upstream end of the line at the injector must also be fed to the injector at a controlled pressure. One of the purposed low-boiling point liquid is a fluorinated hydrocarbon sold under the trademark Freon.sub.114. This liquid, however, has a relatively low vapor pressure on the order of 28 psi (115 cm of Hg). For proper utilization the heat exchanger fluid should be stored at higher that its own vapor pressure. High pressure storage is important because as ambient temperature varies up or down, its vapor pressure follows the same signum direction. With storage at higher than vapor pressure, the liquid pressure can be regulated down to a constant outlet pressure, which will always be higher than vapor pressure at any reasonable ambient temperature. This allows fluid supply to the heat exchanger to be at a constant pressure. It is an object of the present invention, therefore, to provide a system that permits a low-boiling point and low-vapor pressure liquid such as Freon.sub.114 to be utilized accurately in the present invention.
Various fail-safe systems are also provided so that if insufficient pressures develop, the device will shut down rather than yield an incorrect answer.
These objects, as well as others that will be evident in the remainder of the specification, are accomplished by the blood flow monitor of the present invention. It includes a heat exchanger attached to the object to be cooled (i.e. the skin). The heat exchanger operates by having the low-boiling point liquid injected against a thin, conductive surface. Normally, this surface is at approximately skin temperature. When the low-boiling point liquid strikes the surface, it boils, which cools the surface. Cooling ceases when all of the liquid in the heat exchanger boils off. In theory, heat from the skin boils the liquid. This heat flux cools the skin. Because the liquid changes phase, the heat flux from the skin to the liquid is great, and skin cooling is rapid. Surface film boiling heat transfer is manyfold greater than liquid heat transfer and is therefore presumed to be the best system.
The liquid is held in a container, which is connected to the heat exchanger by tubing or other connecting means. In one embodiment, a pressure sensitive valve at the downstream end of the connecting means at the heat exchanger normally blocks the flow of liquid from the source to the heat exchanger except when the pressure of the liquid at the pressure sensitive valve is greater than a predetermined pressure. Control means, which may be high speed valve or a small pump in the connecting means upstream from the pressure sensitive valve, normally blocks the connecting means. In response to a signal, the control means passes or injects a controlled amount of liquid to the line leading to the pressure sensitive valve instantly at a pressure no lower than the predetermined pressure so that the pressure sensitive valve instantly passes the liquid into the heat exchanger where it vaporizes and cools the heat exchanger surface and the skin.
As previously mentioned, the control means for controlling the flow of liquid to the heat exchanger may be a high speed valve that opens and closes very rapidly to allow a precise volume of liquid to pass from the source to the heat exchanger. When the valve opens, the incompressible liquid downstream from the valve is raised to an elevated pressure above the pressure necessary to permit the pressure sensitive valve to open. The pressure sensitive valve opens, and the volume of liquid that flows past the high speed valve also flows past the pressure sensitive valve into the heat exchanger. In another version of this embodiment, the control means is a pump or variable volume injector that receives a controlled volume of liquid and pumps it into the connecting means upon receiving a signal. The pump raises the pressure at the pressure sensitive valve so that the volume of liquid pumped is equal to the volume injected past the valve pressure sensitive valve into the heat exchanger.
In the other embodiment, a miniaturized solenoid valve is mounted within the heat exchanger. The valve normally blocks liquid from entering a small orifice, which when open, allows the liquid to be directly injected by impingement onto the surface of the heat exchanger where boiling takes place. Upstream from the valve, the liquid is maintained at the predetermined pressure by means to be described. By adjusting the period and frequency at which the solenoid-controlled valve stays open, the amount of cooling that the heat exchanger provides can be modified.
The solenoid valve embodiment also has a system for adjusting the solenoid valve without disassemblying the entire heat exchanger.
The source of liquid is a container divided into two compartments by a movable wall. One of the compartments contains the low-boiling point liquid, for example, Freon.sub.114. The other compartment contains a small volume of liquid having a very much higher vapor pressure, and the remainder of the compartment, a much larger volume, contains the gas phases of the liquid. Freon.sub.12, a trademark for a fluorinated hydrocarbon, is one such liquid. The vapor of the higher vapor pressure liquid transmits pressure forces through the movable wall to pressurize the lower vapor pressure liquid to approximately the vapor pressure of the higher vapor pressure gas. As long as some of the higher vapor pressure material in the liquid state remains in its compartment, that compartment will be at equilibrium at the higher pressure. As the volume of the remaining low-boiling point liquid Freon.sub.114 in the other compartment decreases (no gas is present in this compartment as it is above the vapor pressure point) to the point that insufficient liquid may be left for a single test, the system triggers a release valve and all reserve material and its gas will be vented to the atmosphere so that no liquid (in short supply) can be used by the system. Thus, inaccurate readings are avoided on a partially completed test. Furthermore, the automatically emptied container can now be safely shipped for refurbishment and refilling.